At least two reasons:
There are no reports yet of them actually denying anyone service
We already know that if a business puts up a sign that says "If you are gay you cannot eat here," they will lose money (otherwise lots of them would be doing it (and it would be legal)). We don't yet know if putting up a sign saying they won't serve Trump voters (or Hillary voters) would hurt a business. We're about to find out
I could make a case in court the very presence of the sign discrimated against me from eating there.